TRADITIONALLY, bonding wire has been made from gold, prized for its softness and conductivity, but cost pressures are driving the trend towards copper bonding wire.

Wire bonding is used to make interconnections between an integrated circuit and a PCB during device fabrication. Currently, copper bonding wire has a 25 percent share on the market. By 2014, copper is expected to make up 60 to 65 percent of the market.

Singaporean-Israeli company RED Micro Wire is one player in this transition, with heavy R&D in Israel contributing to a new method of manufacturing bonding wire made from copper and glass.

RED Micro Wire is a subsidiary of RED Equipment, which is a turnkey solution provider of secondary semiconductor manufacturing equipment. RED Equipment is also located in Singapore, and it deals with refurbishment of 200mm production equipment for the semiconductor industry.

Due to the shift to 300mm equipment, fabs which require increased capacity on their 200mm lines have to use old equipment, which RED Equipment renovates, upgrades, and provides installation and training services for. It is a high margin business, and the owner of the business decided to invest in RED Micro Wire as a subsidiary.

The company is currently undertaking R&D in Israel, after which the technology will be transferred to Singapore for manufacturing and distribution.

Cheaper, slimmer, protected

Most of the bonding wires today have 18 to 20 micron widths (0.7 to 0.8mm), but Danny Hacohen, VP of business development and marketing with RED Micro Wire, says his company can theoretically scale its wire down to 4 microns, although the industry currently cannot utilise 4 micron wires.

While copper is cheaper than gold, it faces some obstacles to its use as bonding wire.

“Copper wire is completely suitable to fulfil the physics of the material as well as the electrical side. It is almost as good as gold all the needs in assembly and packaging,” said Hacohen.

“The major problem with copper is oxidation and corrosion. Copper wire cannot be exposed to the free air without having a coating. Secondly, the wire itself is very hard and the companies that produce the wires need to find a way to solve it, because the hardness can damage the pad on the wafer.”

One approach used thus far is to use palladium to coat the copper, but RED Micro Wire uses a glass coating on the copper core, resulting in cost savings of 30 percent on the palladium-coated version.

According to Hacohen, the RED Micro Wire product solves the problems of scaling down, corrosion and hardness due to the manufacturing process.

In contrast, RED Micro Wire casts its wires. The automatic machine takes glass and copper raw materials, then heats them up precisely using an induction field, until the materials melt.

This precision is required because the materials have melting points which are close to each other’s. The magnetic field melts both materials within a few seconds of each other, resulting in a soft copper core surrounded by a glass sleeve.

Opportunities and compatibility

A side-effect of a glass-coated copper wire is the insulation afforded by the coating. Currently, wire bonding schemes are subject to restrictions in the distances between wires to prevent shorting faults.“This is a big opportunity for the design people to relax their rules on the design,” Hacohen said. “Theoretically, a wire can touch another wire without shorting out due to the insulation of the glass.”

This could potentially create savings in the back end, as chips can be designed with smaller tolerances and shorter wires.

Hacohen said while it is relatively easy to get a cost saving on the front-end silicon part of devices (by reducing the size of the silicon real estate), manufacturers are looking to break the stalemate on the backend.

“On the backend, for decades, you couldn’t get any cost savings. Obviously, going from gold to copper created the opportunity for cost reductions,” he explained. “In some cases, reducing one cent on the back end allows the company to gain half a percent to one percent on the gross margin of the company. These companies produce millions of chips, and any cost reduction means a lot.”

According to Hacohen, machines and tools do not need to be changed to utilise the glass-coated bonding wire. The company has tested wedge and ball bonding methods, and both have exhibited encouraging results.

Market

Each machine developed by RED Micro Wire can manufacture 25,000km of wire per month. The capacity is rising as the company creates more machines and continues to develop its capabilities. It hopes to gain 9 percent market share by Q2 2013, creating 375,000km of wire a month.

The company is targeting two sets of partners: bonder manufacturers, and integrated device manufacturers who have retained their production capabilities.

Hacohen says one of the major bonder manufacturers is currently testing the material on their manual, semi-automatic and automatic machinery. The current generation of samples has 2 micron thick glass coatings and a 20 micron copper core.

“We are [also] working with all the major integrated device manufacturers (IDMs) in the market,” Hacohen said. “All the top ten assembly houses are interested in our wire.”

“No one is getting exclusivity: we want to penetrate the market. By giving samples to many, we get more data, more problems, more questions, more solutions. We have already delivered two revisions of samples to our partners, and feedback has gone back to R&D, and we are optimising the wire.”

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